1. Field of the Invention
This invention relates to mechanical mountings for zoom lens optical systems of the rear focusing type in which the position of a sharp image of the same object is caused to vary as the focal length varies, and more particularly, to a motor driven focusing mechanism for a compensator lens of such zoom lens with automatic means making it possible to correct deviation of the position of the sharp image from the focal plane.
2. Description of the Prior Art
To the mechanical mountings for zoom lens systems of the type in which when the focal length changes, the position of a sharp image of the same object is caused to move away from the focal plane, there has been the technique of bringing the object image back to the focal plane by moving the lens system as a whole or in part shown in U.S. Pat. No. 3,972,056.
The automatic focusing technique for such deviation correcting mechanism by using a motor built in the lens barrel is also known by Japanese Laid-Open Patent Application No. SHO 58-10706 (corresponding to U.S. patent application Ser. No. 396,030, filed July 7, 1982 now U.S. Pat. No. 4,534,624) and U.S. Pat. No. 4,52,060.
In the zoom lens of the type described above, the higher the focal length, the larger the correcting movement must be made, because the total focusing movement in the telephoto end is longer than in the wide angle end. So, as zooming takes place, the actual value of the focal length must be read out to determine what correcting distance the focusing lens moves. To fulfill such a technical requirement, a novel technical idea is necessarily worked out to a good harmony (structural unity) of zooming and focusing mechanisms since it is difficult to realize it when the conventional structure of motor is employed for incorporation of the focusing mechanism in the lens mounting.
3. Summary of the Invention
A first object of the invention is to provide a mechanical mounting for a zoom lens whose image plane for the same object is caused to change in position when the focal length changes, whereby the deviation of the image plane is corrected by a motor, particularly of the hollow cylindrical form.
In particular, a lens holder containing the one of the zoom components, which also serves as a focusing component, is formed to such a shape as to accommodate the hollow cylindrical motor.
A second object is to provide for the hollow cylindrical motor with means for detecting a change of the focal length so that the number of driving pulses applied to the motor varies continuously or stepwise with the focal length, whereby, on assumption that the motor axially moves the focusing lens to correct the deviation of the image plane by a prescribed distance for every one driving pulse, the correcting movement is varied as a function of the focal length.
In concomitance to the first and second objects, another object of the invention is to provide an arrangement of the aforesaid detecting means between the hollow cylindrical motor and the lens holder that carries the motor which enables the use of the hollow cylindrical type motor to produce a great advantage.
A third object of the invention is to provide a mechanical mounting for a zoom lens optical system of the rear focusing type with a safety mechanism.
In this connection, it should be explained that as the focal length increases, the range of movement of the focusing component increases to overlap the range of movement of the adjacent zoom component thereto, as will be understood from FIG. 5, where interaction between zooming and focusing is shown by a parameter representing the object distance.
That is, the axial position of the focusing lens for the wide angle position differs from that for the telephoto position. Taking a practical example, after the zoom lens has been sharply focused to an object distance of 5 meters in the wide angle position, when zooming is then carried out to the telephoto position, the focusing component has to be moved forward to re-establish the in-focus condition.
Since the automatic focusing control device can, however, be designed in such a way that the corrected focusing movement is superimposed on the zooming movement, there is no possibility of occurrence of a mechanical interference between the focusing component and the adjacent zoom component at any position in the focusing range. The lens system is, on the other hand, designed usually on the assumption that zooming and focusing are operated independently of each other. To avoid accidental interference, therefore, mechanical means must be provided for limiting the total axial movement of the focusing lens depending on the focal length.
In order to selectively operate the automatic and manual focusing modes, the mechanical mounting for such a zoom lens must be provided with a safety mechanism in the form of mechanical stoppers cooperative with the focusing mechanism. Otherwise, if automatic focusing is followed by visual focusing, a mechanical interference between the lens components will often occur. Also, in special photographic situations where the automatic focusing control device cannot be relied on because, for example, the contrast is poor, the lighting is dim, or the actual object distance is shorter than the minimum value of the focusing range of the lens, it will happen that the focusing lens is left beyond the prescribed range of movement thereof, as the focal length decreases with a decrease in the total focusing movement.